Many measuring methods for the acquisition of physical, chemical or electrical quantities have a non-linear relationship between the measured quantity (the output value of the measuring instrument, i.e., an opto-electronic detector) and the measured signal (the input value at the measuring instrument, i.e. the quantity to be measured). In the measuring methods employed in gas analysis, it is frequently not the absolute value of the gas concentration which is determined. Rather, it is the difference in gas concentrations between a reference gas and the measured gas which is identified. Thus, the identification of the differences in gas concentration plays a significant role-in the field of opto-electronic gas analysis.
Non-dispersive gas analyzers are typically designed for the measurement of a permanently selected gas component. The concentration of the gas under measurement is identified by the transmissivity of an infrared light beam as it is beamed through the gas path. Such opto-electronic measuring methods and measuring instruments are described, for example, in brochure number 43-500.01 of Leybold AG. In particular, FIG. 2 on page 2 of the brochure explains the measuring principle of an infrared gas analyzer.
An infrared gas analyzer can primarily be divided into two integrated physical sections. The first section is the measurement side which contains the gas under measurement. The second section is the comparison side which is normally filled with nitrogen and closed in a gas-tight fashion.
Both the measurement side and comparison side are transirradiated with infrared light. An amount of infrared light is absorbed by the gas under measurement, this amount being dependent on the concentration of the gas and occurring within the infrared spectral range. A light chopper wheel that turns at a specified RPM (i.e., 1600 RPM) produces light pulses corresponding to the measurement and comparison beams which transirradiate the measurement and comparison sides respectively. An intensity difference is thereby produced between the beams which corresponds to the concentration difference between the gas under measurement and the comparison gas.
The infrared light pulses which proceed through the measurement side and comparison side impinge an infrared detector which is designationally adjustable to receive a single wavelength range. The output of the detector is supplied to a signal processing means or some other form of electronic evaluator.
There is a non-linear relationship between the infrared detector output and the gas concentration. FIG. 1 illustrates this non-linear relationship. In FIG. 1 the abscissa 10 represents the gas concentration values. The detector output values are represented by the ordinate 11. The non-linear curve 9 represents the relationship between the gas concentration and the output value of the detector. This non-linear curve 9 can be described by the Lambert-Beer Law where the infrared detector is used in an absorption photometer such as described above.
The program line 12 represents the rated line. As with most non-linear devices, the stated object is to correct the non-linear actual curve 9 into the linear rated line 12.
Correction of the curve can take place in several different manners. First, the correction can be carried out within a linearization circuit. Additionally, computers may be used to undertake the correction with the assistance of installed EDP programs. Finally, comparison tables may be used to correct the actual curve 9 into the rated curve 12.
In addition to the absolute identification of the measured quantity, it is of particular importance to obtain the difference between the two absolute quantities. However, the above-described methods for correcting the actual curve into the rated curve cannot be employed when performing a differential measurement. This is due to the fact that different measured signal voltages and curve curvatures derive for an arbitrary but fixed range of differences between the measured quantities, dependent on the underlying reference level.
There are two methods currently employed for calculating the differential measurements. In the first method, the absolute values of the two measured quantities are first ascertained. These two absolute values are subsequently subtracted to determine the difference measurement.
This first method, however, has several disadvantages. It requires exceptional accuracy and precision of the absolute values since the difference to be measured is small when compared to the absolute values. Additionally, this method is disadvantaged in that the two absolute measured quantities must be identified independent of one another. Consequently, any differential value calculated from the two independently measured absolute quantities contains the imprecisions present in two separate measurements.
It also known to employ a method for direct formation of the differences. In such a method, the differential value to be identified must be acquired based on two separate characteristic curves since the influence of the reference level on the measured signal must also be taken into consideration as well as the linearization of the differential measurement. This gives rise to a characteristics field. To this end, the reference level is identified with its own absolute measurement and associated linearization. In order to meet the high precision demands made of the differential measurement, paired testing and balancing agents are usually employed.
When the above-noted methods are employed, the actual range of differential measurements are generally presented without linearization since the influence of the reference level on the linearity of the measured values can only be corrected with great difficulty and an extensive outlay of equipment.